1. Field of the Invention
This invention relates in general to earth boring machines and in particular to a device for applying a downcrowding force onto a rotating cutting tool.
2. Description of the Prior Art
Large diameter relatively shallow holes for foundation piers and the like are often drilled with an auger, either mounted on a truck or on a crane. If on a crane, a rotary table is carried on a platform held by the body and boom of the crane. A pipe known as a kelly is suspended from the boom and lowered through the rotary table. An auger is connected to the bottom of the kelly. The kelly may be square, hexagonal, or round with keys. The rotary table mates with the sides or keys of the kelly to rotate the auger for drilling.
Certain of these units rely on a friction crowd mechanism to apply force to the kelly in addition to kelly weight to advance the auger into the earth. The friction crowd mechanism comprises hydraulic cylinders for moving the rotary bushing downward. Because of the friction between the bushing and the kelly while under torque, the hydraulic cylinders apply a downwardly acting force on the kelly. This force, however, is limited to the friction contact, which may be inadequate for drilling very hard formations.
Certain truck mounted augers utilize a hydraulic cylinder for downcrowding. A sheave is mounted to the top of the kelly for movement therewith. A winch drum is fixedly mounted to the frame. It has two cables wound opposite to each other so that as one is wound in, the other plays out by an equal amount. A series of sheaves are arranged so as to raise or lower the kelly depending on the direction of rotation of the drum. A hydraulic cylinder is mounted in the derrick and through a fixed sheave arrangement will apply a downward force to the kelly sheave when the drum brake is set. While successful, this system requires a mast or derrick capable of withstanding the downcrowding force.
Other types of downcrowding devices are found in the patented art for earth boring machines generally. U.S. Pat. No. 3,717,205 discloses a draw works for an oil and gas well drilling rig. This rig includes a power driven winch drum that has two cables wrapped around it in opposite directions. One cable extends directly to the driving head that rotates the drill pipe, while the other is reeved over the boom and connected to the drilling head. Rotating the drum in one direction moves the drilling head downward, and rotating the drum in the opposite direction moves the drilling head upward. Any downcrowding force must be supplied through the power winch. This requires a large power input for a large downcrowding force, as well as slipping of the clutch to maintain a constant load.